1. Field of the Invention
The present invention relates to a nitride-based semiconductor device having a compound semiconductor layer made of a group III-V nitride-based semiconductor (hereinafter referred to as nitride-based semiconductor) such as GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), BN (boron nitride) or TlN (thallium nitride) or mixed crystal thereof, and a manufacturing method thereof.
2. Description of the Background Art
In recent years, researches have been carried out into GaN-based nitride-based semiconductor devices. In the manufacture of the GaN-based semiconductor device, a substrate of sapphire in the hexagonal system the same as that of GaN is used, since there is no substrate made of GaN. A GaN-based semiconductor layer is grown on the sapphire substrate.
Herein, the lattice constant (a axis) of GaN crystal is 3.19 xc3x85, while the lattice constant (a axis) of sapphire crystal is 4.8 xc3x85. Thus, GaN and sapphire have significantly different lattice constants, and therefore there is strain at a GaN layer grown on the sapphire substrate. The strain causes a large number of lattice defects in the GaN layer. The defect density of the GaN layer grown on the sapphire substrate is about in the range from 1011 to 1012cmxe2x88x922.
The GaN lattice defects described above are transmitted into a semiconductor layer grown on the GaN layer, and therefore a semiconductor device manufactured using a sapphire substrate has a large number of lattice defects. The lattice defects cause current leakage and impurity diffusion in the semiconductor device, and thus adversely affect the characteristics of the semiconductor device.
In a semiconductor laser device with a large number of lattice defects, for example, much leakage current is generated. Therefore, the operation current of the semiconductor laser device increases. As a result, the semiconductor laser device suffers from considerable deterioration, which shortens the useful life of the device.
Therefore, in order to reduce the lattice defects as described above, the following conventional methods have been employed.
According to one method, a GaN-based semiconductor layer is grown through an InGaN layer on a GaN layer grown on a sapphire substrate and having many lattice defects.
As shown in FIG. 11 at (a), the lattice constant (a axis) of InN crystal is 3.55 xc3x85 which is closer to the lattice constant of sapphire than GaN. The method takes advantage of the relation, and the GaN-based semiconductor layer is grown through the InGaN layer having the lattice constant closer to sapphire to reduce the lattice defects. According to the method, however, the lattice defects cannot be reduced sufficiently.
Meanwhile, according to another method, an InGaN/GaN multi-layer film including InGaN films and GaN films alternately layered on one another is formed on a GaN layer grown on a sapphire substrate and having many lattice defects and a GaN-based semiconductor layer is grown through the InGaN/GaN multi-layer film.
In this case, since the lattice constant of InN is closer to that of sapphire than GaN, as shown in FIG. 11 at (b), the average lattice constant of the InGaN/GaN multi-layer film is closer to the lattice constant of sapphire as compared to the case of using only a GaN layer. Also in this case, in the multi-layer structure including a GaN film and an InGaN film, the lattice constant is closer to that of GaN as compared to the case of using only an InGaN layer. The method takes advantage of the relation, and a GaN-based semiconductor layer is grown through an InGaN/GaN multi-layer film to reduce the lattice defects. According to the method, however, the lattice defects cannot be reduced sufficiently.
According to yet another method, an AlGaN/GaN multi-layer film including AlGaN films and GaN films alternately layered on one another is formed on a GaN layer grown on a sapphire substrate and having many lattice defects, and a GaN-based semiconductor layer is grown through the AlGaN/GaN multi-layer film.
In this case, the lattice constant (a axis) of AlN crystal is 3.11 xc3x85 and therefore as shown in FIG. 11 at (c), the average lattice constant of the AlGaN/GaN multi-layer film is smaller than the lattice constant of GaN. According to the method, strain causing lattice defects is concentrated in the AlGaN/GaN multi-layer film to reduce the lattice defects. According to the method, however, the lattice defects cannot be reduced sufficiently.
Meanwhile, according to another method disclosed by Japanese Patent Laid-Open No. 8-56015, an AlGaN/InGaN multi-layer film including AlGaN films and InGaN films alternately layered on one another is formed on a GaN layer grown on a sapphire substrate and having many lattice defects, and a GaN-based semiconductor layer is grown through the AlGaN/InGaN multi-layer film.
According to the method, an AlGaN film is grown on a GaN layer having many lattice defects at a substrate temperature of 800xc2x0 C., and then an InGaN film is grown. Herein, a pair of an AlGaN film and an InGaN film is referred to as one cycle, and a series of forty such cycles altogether are formed. The AlGaN/InGaN multi-layer film thus formed is used to reduce lattice defects extended from the GaN layer.
In the AlGaN/InGaN multi-layer film as described above, however, the AlGaN film is grown first on the GaN layer at a substrate temperature of 800xc2x0 C., and therefore the crystallinity of the AlGaN film formed first in the AlGaN/InGaN multi-layer film is poor. In the AlGaN/InGaN multi-layer film including a plurality of AlGaN films and InGaN films layered on one another, the crystallinity of the film grown first, in other words the crystallinity of the AlGaN film affects the crystallinity of films subsequently formed thereupon. Therefore, the poor crystallinity of the AlGaN film formed first keeps the AlGaN/InGaN multi-layer film from having good crystallinity, and the lattice defects cannot be reduced sufficiently.
Also according to the method described above, the forty cycles of AlGaN and InGaN film pairs are formed, and the AlGaN/InGaN multi-layer film having so many layers of AlGaN films cannot have good crystallinity. Therefore, the lattice defects cannot be reduced sufficiently.
As described above, according to the method disclosed by Japanese Patent Laid-Open No. 8-56015, the characteristics of the semiconductor device can hardly be improved.
Meanwhile, selective lateral growth using a selective growth mask is known as a method of reducing lattice defects. According to the method, a stripe-shaped selective growth mask is formed on a GaN layer grown on a sapphire substrate and having many lattice defects. Then, according to HVPE (Halide Vapor Phase Epitaxy) process, GaN is re-grown on the GaN layer and the selective growth mask. Thus, the lattice defects in the re-grown GaN layer can be reduced to a level of about 6xc3x97107 cmxe2x88x922 in the defect density.
The method using such a selective growth mask is most widely employed for reducing lattice defects in the semiconductor device. At present, nitride-based semiconductor laser devices having a long useful life are provided only by this method.
According to the method, however, a wafer must be once taken out from a crystal growth system to form a selective growth mask thereon, and then the wafer must be returned into the crystal growth system after the selective growth mask is formed. This complicates the process of manufacturing the semiconductor device, which pushes up the manufacturing cost.
It is an object of the present invention to provide a nitride-based semiconductor device having reduced lattice defects and good device characteristics which can be manufactured at a low cost, and a manufacturing method thereof.
A nitride-based semiconductor device according to one aspect of the present invention includes a substrate, a first group III nitride-based semiconductor layer including gallium formed on the substrate, at least one set of layered structures formed on the first group III nitride-based semiconductor layer and including a super lattice multi-layer film and a second group III nitride-based semiconductor layer in this order, and a third group III nitride-based semiconductor layer formed on the at least one set of layered structures and including a device region. The super lattice multi-layer film includes at least one pair of first and second films alternately layered on one another, and the first film is made of a group III nitride-based semiconductor including at least indium and gallium and having a first lattice constant. The second film is made of a group III nitride-based semiconductor including at least aluminum and gallium and having a second lattice constant different from the first lattice constant.
In the super lattice multi-layer film in the nitride-based semiconductor device, the lattice constant of the first film including indium (the first lattice constant) is larger than the lattice constant of the second film including aluminum (the second lattice constant).
In this case, the lattice constants of the first and second films in the super lattice multi-layer film are different, and therefore through defects transmitted to the super lattice multi-layer film from the first group III nitride-based semiconductor layer are subjected to compressive strain and tensile strain in the super lattice multi-layer film and bent in the lateral direction. These laterally bent through defects cancel each other. This strain compensating effect allows through defects to be reduced in the super lattice multi-layer film.
Meanwhile, at the time of forming the super lattice multi-layer film, the first and second films are grown at a low temperature in order to prevent indium in the first film from being liberated. Here, the second film includes aluminum, and therefore the crystallinity of the second film grown at the low temperature is more degraded than that of the first film.
In the super lattice multi-layer film in the nitride-based semiconductor device, at the time of placing the first and second films on one another, the first film having better crystallinity than that of the second film is formed first. Therefore, the crystallinity of the film grown first has a good effect on the crystallinity of films subsequently layered thereon. Therefore, the crystallinity can be improved in the super lattice multi-layer film.
As in the foregoing, lattice defects are reduced in the super lattice multi-layer film and good crystallinity results, so that the second group III nitride-based semiconductor layer formed on the super lattice multi-layer film may have improved crystallinity.
In the nitride-based semiconductor device, the third group III nitride-based semiconductor layer including a device region is formed on the second group III nitride-based semiconductor layer with improved crystallinity, and therefore good crystallinity results in the third group III nitride-based semiconductor layer, particularly in the device region. Thus, in the nitride-based semiconductor device, the device characteristics can be improved.
In the nitride-based semiconductor device, forming the super lattice multi-layer film allows the crystallinity to be improved without using a selective growth mask. Therefore, the manufacture is easier, and the manufacturing cost can be reduced accordingly.
At least one set of layered structures includes multiple sets of layered structures placed on the first group III nitride-based semiconductor layer, and the second group III nitride-based semiconductor layers included in the multiple sets of layered structures preferably have the same composition or different compositions. When multiple sets of layered structures are placed on one another, lattice defects which cannot be reduced in a single layered structure can be reduced in overlying layered structures formed thereon. Thus, the lattice defects can effectively be reduced.
The average lattice constant of the first and second films in the super lattice multi-layer film is preferably substantially equal to the lattice constant of the first group III nitride-based semiconductor layer. In such a super lattice multi-layer film, the strain compensating effect described above is increased, and therefore the lattice defects can more effectively be reduced.
The second group III nitride-based semiconductor layer may include gallium. The first group III nitride-based semiconductor layer may be made of GaN, the second group III nitride-based semiconductor layer may be made of GaN, the first film in the super lattice multi-layer film may be made of InGaN, and the second film in the super lattice multi-layer film may be made of AlGaN.
In this case, InGaN has a larger lattice constant than that of GaN, and therefore the first film in the super lattice multi-layer film has compressive strain. Meanwhile, AlGaN has a smaller lattice constant than that of GaN, and therefore the second film in the super lattice multi-layer film has tensile strain. In the super lattice multi-layer film including the first film with such compressive strain and the second film with such tensile strain layered on one another, through defects transmitted from the first group III nitride-based semiconductor layer are subjected to the compressive strain and the tensile strain and bent in the lateral direction. These laterally bent through defects cancel each other. This strain compensating effect allows the through defects to be reduced in the super lattice multi-layer film.
In the super lattice multi-layer film, preferably, the In composition ratio in the first film is 5%, and the Al composition ratio in the second film is 25%. In the super lattice multi-layer film, the average lattice constant of the first and second films is substantially equal to the lattice constant of the first group III nitride-based semiconductor layer. Therefore, in this super lattice multi-layer film, the strain compensating effect is increased, so that the lattice defects can more effectively be reduced.
In the super lattice multi-layer film as described above, the thickness of each of the first and second films is preferably in the range from 5 xc3x85 to 70 xc3x85, more preferably in the range from 5 xc3x85 to 40 xc3x85.
The at least one pair of the first and second films is preferably at least two pairs and at most nine pairs, more preferably at least three pairs and at most six pairs.
In the super lattice multi-layer film including the described number of the first and second films having the described thickness, lattice defects can effectively be reduced.
In particular, the number of the second films made of AlGaN is set in the range from two to nine, more preferably three to six, so that in the super lattice multi-layer film, crystallinity degradation caused by layering too many such AlGaN films does not result. Therefore, the crystallinity can be improved in the super lattice multi-layer film, and the lattice defect reducing effect can be increased.
The at least one set of layered structures preferably includes three sets of layered structures. In the arrangement of such three sets of layered structures, lattice defects which cannot be reduced in a single layered structure can effectively be reduced.
A method of manufacturing a nitride-based semiconductor device according to another aspect of the present invention includes the steps of forming a first group III nitride-based semiconductor layer including gallium on a substrate, forming at least one set of layered structures including a super lattice multi-layer film and a second group III nitride-based semiconductor layer in this order on the first group III nitride-based semiconductor layer, and forming a third group III nitride-based semiconductor layer having a device region on the at least one set of layered structures. The step of forming the at least one set of layered structures includes the step of forming the super lattice multi-layer film by forming at least one pair of first and second films alternately layered on one another, and the first film is made of a group III nitride-based semiconductor including at least indium and gallium and having a first lattice constant. The second film is made of a group III nitride-based semiconductor including at least aluminum and gallium and having a second lattice constant different from the first lattice constant.
According to the method of manufacturing a nitride-based semiconductor device, a super lattice multi-layer film including a first film and a second film layered on one another and having different lattice constants is formed on the first group III nitride-based semiconductor layer. In this case, in the super lattice multi-layer film, the lattice constant of the first film including indium (the first lattice constant) is larger than in the lattice constant of the second film including aluminum (the second lattice constant). Therefore, through defects transmitted from the first group III nitride-based semiconductor layer to the super lattice multi-layer film are subjected to compressive strain and tensile strain in the super lattice multi-layer film and bent in the lateral direction. These laterally bent through defects cancel each other. The strain compensating effect allows the through defects to be reduced in the super lattice multi-layer film.
Meanwhile, in the step of forming the super lattice multi-layer film as described above, the first and second films are grown at a low temperature in order to prevent indium in the first film from being liberated. When the second film including aluminum is grown at the low temperature, the crystallinity of the second film is degraded.
Therefore, by the method according to the present invention, at the time of forming the super lattice multi-layer film, the first film having better crystallinity than the second film is grown first. Therefore, the good crystallinity of the film grown first has a good effect upon the crystallinity of films subsequently layered thereon. Therefore, in the super lattice multi-layer film, the crystallinity can be improved.
As described above, lattice defects can be reduced and good crystallinity results in the super lattice multi-layer film, so that the crystallinity can be improved in the second group III nitride-based semiconductor layer formed on the super lattice multi-layer film.
According to the method of manufacturing a nitride-based semiconductor device described above, the third group III nitride-based semiconductor layer including the device region is formed on the second group III nitride-based semiconductor layer with improved crystallinity. Therefore, good crystallinity results in the third group III nitride-based semiconductor layer, particularly in the device region. Therefore, a nitride-based semiconductor device having good device characteristics can be manufactured.
According to the method of manufacturing a nitride-based semiconductor device described above, forming the super lattice multi-layer film allows the crystallinity to be improved without using a selective growth mask. Therefore, the manufacturing process is easier and the manufacturing cost can be reduced accordingly.
The step of forming the at least one set of layered structures includes the step of placing multiple sets of layered structures on the first group III nitride-based semiconductor layer. The second group III nitride-based semiconductor layers included in the multiple sets of layered structures preferably have the same composition or different compositions.
Thus, when the multiple sets of layered structures are placed on one another, lattice defects which cannot be reduced in a single layered structure can be reduced in overlying layered structures formed thereon. As a result, the lattice defects can effectively be reduced.
The step of forming at least one set of layered structures preferably includes the step of setting the same composition for the first and second films in the super lattice multi-layer film so that the average lattice constant of the first and second films is substantially equal to the lattice constant of the first III nitride-based semiconductor layer. Therefore, the strain compensating effect described above is increased, and lattice defects can more effectively be reduced.
The step of forming at least one set of layered structures may include the step of forming the second group III nitride-based semiconductor layer including gallium.
Furthermore, the step of forming the first group III nitride-based semiconductor layer may include the step of forming the first group III nitride-based semiconductor layer made of GaN. The step of forming at least one set of layered structures may include the step of forming the second group III nitride-based semiconductor layer made of GaN and the step of forming at least one pair of the first film made of InGaN and the second film made of AlGaN to form the super lattice multi-layer film.
In this case, InGaN has a lattice constant larger than that of GaN, and therefore the first film in the super lattice multi-layer film has compressive strain. Meanwhile, AlGaN has a lattice constant smaller than that of GaN, and therefore the second film in the super lattice multi-layer film has tensile strain. In the super lattice multi-layer film including the first film with the compressive strain and the second film with the tensile strain layered on one another, through defects transmitted from the first group III nitride-based semiconductor layer are subjected to the compressive strain and the tensile strain and bent in the lateral direction. These laterally bent through defects cancel each other. This strain compensating effect allows through defects in the super lattice multi-layer film to be reduced.